power: pmic: sunxi: add SPL support for the AXP803

power: pmic: sunxi: add SPL support for the AXP803

The AXP803 has been around for about a decade now, but so far we didn't
need SPL support, since the DRAM rail was wired up correctly at reset.

Now some boards using the A133 SoC use the (compatible) AXP707 with DDR4
memory, which requires the SPL to set the required 1.1V voltage manually.

Add the descriptions for the DC/DC regulators of the AXP803, and enable
that when CONFIG_AXP803_POWER is enabled.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>